A conventionally, noise reduction circuit for television receivers utilizing frame correlation is well understood. In general, signal components, i.e., video components, have a high correlation between successive frame video signals. Noise components, however, have a low correlation between successive frame video signals. Typically, the frame difference signal representing the difference between the video components of two successive frame video signals is almost zero for a static picture, and is a relatively large value for a changing picture or a picture-in-motion. The frame difference signal representing the difference between the noise components of two successive frame video signals is small for both a static picture and a picture-in-motion. Thus, a low level frame difference signal can be assumed to represent noise included in the frame video signals.
Therefore, a typical noise reduction circuit extracts a low-level frame difference signal from the frame video signal. Thus, the noise component of the video signal corresponding to the frame difference signal of low level is removed from the video signal by subtraction. In such a noise reduction circuit, a frame memory is used for establishing a delayed one of the successive frame video signals. The frame memory stores the input video signal for a period corresponding to the time between successive frames of the first and second color difference signals.
Further, such a conventional noise reduction circuit has a decoding circuit and a pair of noise reduction circuits. The decoding circuit decodes two color difference signals (for example, color difference signals R-Y and B-Y ) from the chrominance signals. Hereupon, the color difference signals R-Y and B-Y are transmitted together with a luminance signal Y in place of three color signals, i.e., in place of a red color signal R, a green color signal G and a blue color signal B in an NTSC television system. Television receivers then reproduce the three color signals R, G and B from the luminance signal Y and the color difference signals R-Y and B-Y . The color difference signals R-Y and B-Y represent difference signals between the red color signal R and the luminance signal Y and the the blue color signal B and the luminance signal Y.
Each of the noise reduction circuits receives the color difference signal R-Y or B-Y as its one input signal to carry out noise reduction and also receives a frame difference signal between successive frame signals of the color difference signal R-Y or B-Y , when the level of the frame difference signal is lower than a prescribed level.
In more detail, the frame difference signals are converted to corresponding compensation signals by converting circuits, as described later. The compensation signals are subtracted from the corresponding color difference signals. Thus, noise in the color difference signals are reduced.
The conventional noise reduction circuit carries out the noise component extracting operation, i.e., the noise reduction, when the frame difference signal is lower than a prescribed level. The circuit interrupts the noise component extracting operation so that the noise reduction is not carried out when the frame difference signal is higher than the prescribed level.
Such a conventional noise reduction circuit, however, has a drawback, as described below.
In an example of changing of signals in the picture-in-motion, a state occurs in which one frame difference signal between two successive signals of first color difference signal, e.g., R-Y , is lower than a prescribed level and another frame difference signal of second color difference signal, e.g., B-Y , is higher than the prescribed level.
FIG. 1 shows such a typical example. in FIG. 1, vector Ca represents a prior chrominance signal prior to a period of one frame. Vector Cb represents a present chrominance signal at a present time. Both the prior chrominance signal Ca and the present chrominance signal Cb are successive signals of chrominance signals C.
Vector C1b represents a present first frame color difference signal corresponding to the first color difference signal, e.g., R-Y , of the present chrominance signal Cb.
Vector C1a represents a prior first frame color difference signal corresponding to the first color difference signal R-Y of the prior chrominance signal Ca. The prior first frame color difference signal C1a is obtained by decoding the prior chrominance signal Ca and delaying the decoded signal by the period of one frame through a frame memory.
Vector C2b represents a present second frame color difference signal corresponding to the second color difference signal, e.g., B-Y , of the present chrominance signal Cb.
Vector C2a represents a prior second frame color difference signal corresponding to the second color difference signal B-Y of the prior chrominance signal Ca. The prior second frame color difference signal C2a is obtained by decoding the prior chrominance signal Ca and delaying the decoded signal by the period of one frame through another frame memory.
A signal component e1 represents a first frame difference signal between the prior first frame color difference signal C1a and the present first frame color difference signal C1b.
Another signal component e2 represents a second frame difference signal between the prior second frame color difference signal C2a and the present second frame color difference signal C2b. Hereupon, the first frame difference signal e1 is lower than a prescribed level, and the second frame difference signal e2 is higher than the prescribed level.
As shown in FIG. 1, the chrominance signal C changes from the prior chrominance signal Ca to the present chrominance signal Cb during the period of one frame. Thus, one frame difference signal, e.g., the first frame difference signal e1, is lower than the prescribed level and the other frame difference signal, e.g., the second frame difference signal e2, is higher than the prescribed level.
FIG. 2 shows an example of the noise extraction characteristic R of a typical converting circuit, a described before. In such a converting circuit, a frame difference signal between two successive color difference signals is converted to a corresponding noise component in accordance with the noise extraction characteristic R. In FIG. 2, the symbol e1 corresponds to the first frame difference signal e1 in FIG. 1. The symbol x represents the prescribed level. The symbol eN1 represents a compensation signal converted from the first frame difference signal e1 in accordance with the noise extraction characteristic R, as described above. In FIG. 2, the symbol e2 also corresponds to the second frame difference signal e1 in FIG. 1. The symbol eN2 represents a compensation signal converted from the second frame difference signal e2 in accordance with the noise extraction characteristic R.
When such a frame difference signal e2 with a level higher than a prescribed level is obtained, the conventional converting circuit interrupts the converting operation. For example, second frame difference signal e2 with the level higher than the prescribed level is not converted to a corresponding compensation signal.
In the conventional noise reduction circuit for chrominance signal, however, both the noise reduction circuits or the converting circuits corresponding to the respectives of two color difference signals carry out the noise reduction operation independently.
As a result, the noise reduction for the first color difference signal is carried out, but the noise reduction for the second color difference signal is interrupted, as shown in the above example. In other words, the conventional noise reduction circuit may carry out the noise reduction, in spite of a considerable change occuring in two successive video signals due to a typical picture-in-motion. This causes a defect in that displayed images are affected with an incompatibility such as a residual image, etc.